A hydroelectric power generation system includes a water turbine disposed in a channel that carries a flow of a fluid, a generator driven by the water turbine, and a controller that performs a first control. The channel includes a first channel located on an inflow side of the water turbine. The controller controls, in the first control, a flow rate or a head of the water turbine so that any one of a pressure of the fluid in the first channel, a flow rate of the fluid in the first channel, and a liquid level of the fluid in a first reservoir from which the fluid flows out to the first channel approaches a first target value.